1. Field of the Invention
The present invention relates to a quinoxaline derivative, and a light-emitting element, a light-emitting device, and an electronic device each using the quinoxaline derivative.
2. Description of the Related Art
By using organic compounds, compared with inorganic compounds, a wider variety of structures can be taken and materials having a variety of functions depending on the molecular design can be synthesized. Because of these advantages, photo electronics and electronics each using functional organic materials have been attracting attention in recent years.
As examples of electronic devices using organic compounds as functional organic materials, there are solar cells, light-emitting elements, organic transistors, and the like. These examples are devices using electric properties and optical properties of such organic compounds. In particular, light-emitting elements have been developing remarkably.
Alight emission mechanism is said to be as follows: by applying a voltage to a pair of electrodes with a light-emitting layer interposed therebetween, an electron injected from a cathode and a hole injected from an anode recombine with each other in an emission center of the light-emitting layer to form a molecular exciton, and the molecular exciton releases energy in returning to a ground state; accordingly, light is emitted. As excited states, a singlet excited state and a triplet excited state are known. It is thought that light emission can be obtained through either a singlet excited state or a triplet excited state.
Such light-emitting elements have a lot of material-dependant problems for improvement of element characteristics. In order to solve the problems, improvement of element structures, development of materials, and the like have been carried out.
As the most basic structure of a light-emitting element, the following structure is known: a hole-transporting layer made of an organic compound having a hole-transporting property and an electron-transporting light-emitting layer made of an organic compound having an electron-transporting property are stacked to form a thin film of approximately 100 nm thickness in total, and this thin film is interposed between electrodes (see Non-Patent Document 1: C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
By applying a voltage to the light-emitting element described in Non-Patent Document 1, light emission can be obtained from the organic compound having a light-emitting property and an electron-transporting property.
Further, in the light-emitting element described in Non-Patent Document 1, functions are separated so that the hole-transporting layer transports holes whereas the electron-transporting layer transports electrons and emits light. However, a variety of interactions (e.g., exciplex formation) frequently occur at an interface of stacked layers. As a result, a change in emission spectrum or a decrease in emission efficiency may occur.
In order to suppress a change in emission spectrum or a decrease in emission efficiency due to interactions at an interface, a light-emitting element having further functional separation has been developed. For example, a light-emitting element in which a light-emitting layer is interposed between a hole-transporting layer and an electron-transporting layer has been proposed (Non-Patent Document 2: Chihaya Adachi et al., Japanese Journal of Applied Physics, vol. 27, No. 2, L269-L271 (1988)).
For a light-emitting element as described in Non-Patent Document 2, in order to more effectively suppress interactions occurring at an interface, a light-emitting layer is preferably made of a bipolar organic compound having both an electron-transporting property and a hole-transporting property.
However, most organic compounds are monopolar materials having either a hole-transporting property or an electron-transporting property.
Therefore, development of bipolar organic compounds having both an electron-transporting property and a hole-transporting property has been required.
In Patent Document 1 (PCT International Publication No. 2004/094389), a bipolar quinoxaline derivative is disclosed. However, its characteristics are not yet satisfactory, and development of a wider variety of bipolar organic compounds has been required.